1. Field of the Invention
The invention relates generally to methods of measuring coating thicknesses. More specifically, the invention relates to the field of using eddy current measurements to make the nondestructive thickness determination.
2. Description of the Related Art
Unlike metals, composite materials are easily damaged in the event of lightning strike, since most of them are either nonconductive (e.g., fiberglass), or low in conductivity (e.g., carbon), which do not allow the energy to follow easily from the impact to some point of dissipation. As a result, a lightning strike can cause detrimental effects in composite materials and structures, such as airframe, turbine engine components and wind turbines. Structural damage can include overheating, vaporization of resin or possible burn-through the laminate, as well as aircraft electrical system malfunction or permanent damage due to electromagnetic field interference.
It is common practice to bond conductive mesh or foil made of high-conductivity metals (Al, Cu, Ni-coated, etc.) to the composite structure as the outside ply to be used as current exit routes to discharge lightning quickly. An example of a mesh-equipped composite skin after lightning strike can be seen in FIG. 1. A multilayer coating system (e.g., surfacer, primer, and topcoat) is applied to composite materials above this conductive layer to improve surface properties, including appearance, adhesion, wear resistance, moisture protection, etc. This multilayer coating system is a critical part of the quality assurance and cost control process. In particular, the applied coating thickness must be controlled in certain range to achieve the required lighting strike protection. In other words, there is a need to conduct accurate non-conductive coating thickness measurement in composite from the point of view of lighting strike protection and flight safety.
Various techniques have been used in the industry to measure thickness of this multilayer coating system on conductive mesh/foil substrates in composite materials, including capacitive, microwave and ultrasonics. For example, ultrasonic testing (UT) has been extensively used to measure the thickness of the coating on composite material by exploiting its high sensitivity to acoustic impedance difference between the coating system and it's underlying composite substrate. However, this excellent sensitivity also causes difficulty in signal interpretation as each coating interface (e.g., surfacer/substrate, surfacer/primer, etc.) generates an ultrasonic signal. Consequently, special training, certification and extensive UT experience is needed for operators to interpret the UT signal for coating thickness measurement, which in turn increases inspection cost and time. The difficulty in interpreting the complicated ultrasonic reflections can potentially lead to coating thickness measurement error and insufficient lightning protection. Further, UT technique is difficult to be deployed in production and field because of the need to use couplant to transmit UT into the part.